1. Field of the Invention
The present invention relates to the improvements of an intake-air quantity control apparatus for an internal combustion engine equipped with a variable valve timing system capable of electronically arbitrarily controlling an intake- and/or exhaust-valve timing, depending upon engine/vehicle operating conditions, and specifically to techniques for controlling a quantity of intake air entering an internal combustion engine toward a desired intake-air quantity by electronically controlling an intake valve open timing (often abbreviated to "IVO") and an intake valve closure timing (often abbreviated to "IVC").
2. Description of the Prior Art
Recently, there have been proposed and developed various electronically-controlled variable valve timing systems which are capable of operating intake and exhaust valves electromagnetically. One such electronically-controlled variable valve timing system for an internal combustion engine having electromagnetically-operated valve units has been disclosed in Japanese Patent Provisional Publication No. 9-256823. In the Japanese Patent Provisional Publication No. 9-256823, each of intake and exhaust valves is comprised of an electromagnetic solenoid valve whose opening and closing are achieved by way of an electromagnetic force instead of the use of a typical cam-drive mechanism. Thus, an intake-valve closure timing (IVC), an intake-valve open timing (IVO), an exhaust-valve open timing (EVO), and an exhaust-valve closure timing (EVC) can be continually changed in response to command signals from an electronic control unit (ECU). In such internal combustion engines with a variable valve timing control system having electromagnetically-operated valve units, an intake-air quantity can be adjusted by properly controlling or managing an intake valve timing (an intake-valve open timing and/or an intake-valve closure timing), in place of throttle-opening adjustment. In this type of engines with electromagnetically-operated engine valve units, a throttle valve is often eliminated, or a throttle valve is installed on the engine only for the purpose of generation of a negative pressure in an intake-air passage. Suppose an internal pressure in the intake-air passage reaches a pressure level close to atmospheric pressure with the throttle kept at an extremely less throttle opening. In this case, the intake-air quantity control system based on adjustment of an intake-valve opening time period (a time interval between IVO and IVC) is superior to that based on only the throttle-opening adjustment, from the viewpoint of a pumping loss.